Metal uptake processes

Metal sorption appears to concentrate primarily in the myco-biont. Thus, understanding the metal uptake process in free-living fungi provides crucial data that substantially inform lichen studies. Gadd (1993) provides a thorough review of studies investigating metal uptake by... 

The logic of the evolution of insertion can now be considered. Much as the most primitive selectivity of the chemistry of the uptake process, pumping, carrying and final binding to form a useful enzyme is not an invention by organisms but is a necessary consequence of inevitable equilibrium considerations (see Section 4.17), so the binding of particular metal ions to particular chelatase proteins was similarly selected... 

As mentioned previously, siderophores must selectively bind iron tightly in order to solubilize the metal ion and prevent hydrolysis, as well as effectively compete with other chelators in the system. The following discussion will address in more detail the effect of siderophore structure on the thermodynamics of iron binding, as well as different methods for measuring and comparing iron-siderophore complex stability. The redox potentials of the ferri-siderophore complexes will also be addressed, as ferri-siderophore reduction may be important in the iron uptake process in biological systems. 

Uptake of small organic metal complexes over transport systems of organic metabolites may be possible, for example, of small organic acids like citrate or amino acids. However, only few examples of such processes have been studied so far. Increased uptake of cadmium by an alga has been observed in the presence of citrate and has been attributed to accidental transport of the metal-citrate complex over a citrate transporter [212]. Transport systems of inorganic anions may also play a role in metal transport. Silver uptake by algae was enhanced in the presence of thiosulfate. In this case, the silver thiosulfate complex was transported over a sulfate uptake system [213]. It remains to be demonstrated how widespread these processes may be for metal uptake in the aquatic environment [12]. 

It is perhaps wise to begin by questioning the conceptual simplicity of the uptake process as described by equation (35) and the assumptions given in Section 6.1.2. As discussed above, the Michaelis constant, Km, is determined by steady-state methods and represents a complex function of many rate constants [114,186,281]. For example, in the presence of a diffusion boundary layer, the apparent Michaelis-Menten constant will be too large, due to the depletion of metal near the reactive surface [9,282,283], In this case, a modified flux equation, taking into account a diffusion boundary layer and a first-order carrier-mediated uptake can be taken into account by the Best equation [9] (see Chapter 4 for a discussion of the limitations) or by other similar derivations [282] ... 

Bioaccumulation is a complicated process that couples numerous complex and interacting factors. In order to directly relate the chemical speciation of an element to its bioavailability in natural waters, it will be necessary to first improve our mechanistic understanding of the uptake process from mass transport reactions in solution to element transfer across the biological membrane. In addition, the role(s) of complex lability and mobility, the presence of competing metal concentrations and the role(s) of natural organic ligands will need to be examined quantitatively and mechanistically. The preceding chapter... 

Holten and coworkers have also shown that 4-coordinate Ni-porphyrin species existing in coordinating solvents are induced to pick up ligands by laser excitation (45-47). This ligand-uptake process can also be understood in terms of the d-electron configuration of the metal. For the Ni-porphyrin molecules that have no axial ligands, the (d 2 ) state is the groun state. 

Unlike desferrioxamine analogs designed for specific therapeutic purposes described above, chiral DFO analogs that form conformationally unique complexes with iron(lll) were designed to serve as chemical probes of microbial iron(lll) uptake processes. As mentioned above, ferrioxamine B can form a total of five isomers when binding trivalent metal ions, each as a racemic mixture. Muller and Raymond studied three separate, kinetically inert chromium complexes of desferrioxamine B (N-cis,cis, C-cis,cis and trans isomers), which showed the same inhibition of Fe-ferrioxamine B uptake by Streptomyces pilosus. This result may indicate either that (i) ferrioxamine B receptor in this microorganism does not discriminate between geometrical isomers, or that (ii) ferrioxamine B complexes are conformationally poorly defined and are not optimal to serve as probes. 

The recent detailed investigations on Zn(II) and Pb(II) have advanced our understanding of heavy metal cation binding to C-S-H. However there are still a number of important questions that require attention. We need to understand the relationship, if any between Ca Si ratio and metal uptake. We also need to understand how to interpret the sorption process, whether as a solid solution (Tommaseo Kersten 2002) or as precipitation within the C-S-H particles or as a sorption process suggested by Glasser (1993) to the surfaces of the crystalline domains. 

The metal ion uptake profiles are shown in Fig. 11.1 for variations of NaCNS concentration (Fig. 11.1a), temperature (Fig. 11.1b) and plasticisation drawing (Fig. 11.1c) of the precipitation bath for Co uptake. Similar curves were obtained with Ni. Table 11.2 shows the data for different parameters related to a fully metallised fibre obtained after metallisation of PAN fibres, produced under different experimental conditions of the precipitation bath. Despite the fact that the uptake profiles are considerably different and the data obtained (diffusion coefficient) confirms this, no remarkable changes are observed in the total amount of metal absorbed by the fibre. This means that saturation for metal uptake is obtained independently of the precipitation bath parameters. The role of these parameters is limited to the rate of metal uptake, and a choice for the optimal value of these parameters should be based on economic reasons first the consumption of chemicals and energy and, secondly, the processing time. Taking these two criteria into account, a NaCNS concentration of about 12%, a temperature of 283 K and a plasticisation drawing of 500% are further used. 

The simple definition of biological availability as the fraction of the total trace metal available for uptake by the biota implicitly assumes that all relevant organisms will have similar uptake characteristics. In addition the use of biological availability as a chemical parameter involves the assumption that it can be identified with particular chemical species or groups of species. Perhaps surprisingly many of the studies carried out to date have lent support to these assumptions, though the detailed mechanisms of the uptake processes remain unclear. Some of the inorganic chemical species identified as... 

Keywords Biosorption Activated sludge process Removal of heavy metals Sequential extraction of metals Metal uptake... 

The invaluable comments of the referees are highly acknowledged in improving the quality of this contribution and for encouraging the authors to continue a more detailed experimental study, which will clarify the role of extracellular polymer substances in the mechanism of metal uptake by activated sludge process. 

Before dyeing with oxidation dyes, the furs are treated with the appropriate killing agents and then mordanted with metal salts. Iron, chromium, and copper salts, alone or in combination, are used for mordanting, and the uptake process requires several hours. Adjustment of the pH is effected with formic, acetic, or tartaric acid. The final dyeing process is carried out in paddles with the precursors and hydrogen peroxide until the actual dye lake is developed and adsorbed within the hair fiber. It takes quite a few hours at room temperature until the dyeing process is finished. 

The lateral and vertical distributions of these carrier-phase metals in estuaries are largely controlled by particle dynamics, as opposed to other metals (e.g., Cu, Zn, and Co) which will be more affected by biotic uptake processes. 

Early studies of metal uptake by lichens presented metal sorption as an ion exchange process, and reported metal accumulation in terms of relative uptake capacity, or the capacity of one metal to displace another as an adsorbed complex. Competitive uptake studies by Puckett et al. (1973)... 

---